Multidirectional symbol readers such particularly suitable for use in the industrial field, in a fixed position in front of a conveyor on which objects, to which bar codes are affixed, pass by.
For such industrial applications it is in fact necessary to use omnidirectional scanners because the bar codes affixed to the objects can have different orientations, in particular extend either horizontally or vertically. Furthermore, the scanners should permit instantaneous capture of bar codes in order to allow relatively high conveying speeds for the objects. Finally, these scanners should have bar code scanning modes which guarantee systematic capture of each code. In fact, the object to be identified passes by on a conveyor and therefore appears only once in front of the scanner, which excludes any risk of scanning error such as permitted for example at points of sale where the object can be passed again in front of the scanner.
Currently, three main types of multidirectional scanners are used in the industrial field.
The first type of scanner, such as described in particular in the patents U.S. Pat. No. 5,464,972 and EP-697 674, is basically designed to deliver a beam towards a rotating mirror having one or more facets, which deflect said beam towards a number of fixed mirrors partially or completely surrounding the rotating mirror. As explained in these patents, this design enables omnidirectional scanning to be obtained, consisting of a number scanning lines extending with different orientations. The main drawback of this type of scanner lies in the fact that non-optimized, pseudo-random scanning is obtained, which imposes relatively low conveying speeds with a view to ensuring systematic scanning of each bar code.
The second type of scanner, such as described in the patent EP-323 026, basically comprises a first rotating mirror, the normal of which is inclined by several degrees with respect to the axis of rotation, designed to deliver a cone-shaped beam towards a second mirror of identical design, rotated at lower speed than that of the first mirror. This design allows omnidirectional scanning in the shape of a rosette to be obtained, which nevertheless results in the same disadvantages as those of the aforementioned first type of scanner.
Finally, the third type of scanner, such as described in particular in the U.S. Pat. No. 5,371,347 and U.S. Pat. No. 5,187,612, basically consists of a gyratory scanning head carrying a mirror, and arranged so as to oscillate in two perpendicular planes and to rotate about the axis of intersection of said planes. Such scanners enable the drawbacks of the aforementioned scanners to be overcome and offer good scanning reliability at relatively high conveying speeds. However, they require specific basic parts of delicate manufacture bordering on the field of horology, and therefore at a high cost price.